This invention relates to a large-screen projection-type display, and particularly to mounting structure means for a transmission-type screen of about 110-inch diagonal length.
FIG. 1 is a cross-sectional diagram of a conventional projection-type display. In FIG. 1, there are shown a projection source 1 such as a CRT or a liquid crystal panel, a projection lens 2, and a screen 3. In addition, there are shown a front sheet 4 of about 1-mm thickness, and a Fresnel sheet of about 10-mm thickness. This screen is about 110 inches in diagonal length, 2.2 m wide and 1.66 m high, and is made chiefly of an acrylic resin material. This screen is normally mounted on the wall of a hall of about 3-m ceiling height so that the lower end of the screen is located about 1.2 m from the floor. Thus, it is suited to present image information to many people.
The detailed optical construction of this screen is described in U.S. Pat. No.4536056 which our inventors filed.
The conventional projector screen, as shown in FIG. 1, is vertically constructed and mounted and thus has the following drawbacks.
(1) The screen is normally desired to be tilted forward about 7.degree. considering the limited vertical directivity (about .+-.15.degree. ) of the screen, but if it is done so, the front sheet 4 is deformed by its own weight as shown in FIG. 2. Consequently, a large gap is caused between the Fresnel sheet and the front sheet, preventing the image from being focused.
(2) Even when the screen is vertically mounted as shown in FIG. 1, the left side (light-incident surface) and right side (light-exiting surface)
of the Fresnel sheet 5 causes a warp .delta. by the so-called bimetal effect as shown in FIG. 3 if their temperatures are high and low, respectively.
As a typical example, we now consider the case where the periphery of the screen shown in FIG. 1 is surrounded by an adiabatic wall and half the power of about 1000 W of the projection source 1, or 500 W is radiated to viewers 6 through the screen as shown in FIG. 2. From the formula of thermal conduction, the following equation is derived: EQU 500W={.sigma..multidot.(2200mm).multidot.(1660mm).multidot..DELTA.T}/(10mm) (1)
where
.sigma. .apprxeq. 0.2 mW/mm .degree. C. (thermal conductivity) PA0 .DELTA.T : the temperature difference between the surface and back of the Fresnel sheet 5 PA0 .thrfore. .DELTA.T =6.8.degree. C.
On the other hand, the linear expansion coefficient (.alpha.) of acryl is about 70 PPM/.degree. C. and thus the radius of curvature, R of the bimetal Fresnel sheet due to the temperature difference is given by EQU R=t/.epsilon.=t/(.alpha..DELTA.T)=(10mm)/(476PPM) (2)
where t =10 mm (thickness)
The relation of the radius of curvature, R to the distortion .delta..sub.1 (FIG. 3) is given by EQU .delta..sub.1 .apprxeq..alpha..sup.2 /(2R).apprxeq.(.alpha..DELTA.Ta.sup.2)/(2t) (3) EQU .apprxeq.29mm (4)
The value of 29 mm is obtained by substituting 1100 mm (half width of the screen) into a, or the mean radius from the screen center, of the above equation.
In the general CRT projection-type display using three CRTs, in order that the change of color displacement among three colors on the screen is restricted to within about 0.5 pixel, the change of .delta..sub.1 is required to be limited to within about 7 mm. Therefore, the prior art has the drawback of causing color displacement due to the change of the temperature difference between the surface and back of the sheet.
The response of the thick Fresnel sheet 5 to the humidity change of the ambient environment is as long as several months, and thus quite stable. However, since the response of the thin front sheet 4 to the humidity change is as short as several days, it has a problem of causing a warp .delta..sub.2 as shown in FIG. 3. FIG. 3 shows the case when the Fresnel sheet is vertically mounted.
Moreover, since the thickness of the Fresnel sheet 5 is as large as about 10 mm, the mass of the screen is as large as about 53 kg even except for the screen frame. Therefore, the whole display becomes heavy.
There is another prior art of using a single sheet for constructing the screen. In this case, the screen can be tilted forward about 7.degree., but regrettably the peripheral corners of the screen are deficient in the relative amount of light.
In addition, there is a conventional two-sheet screen of the type in which the Fresnel sheet 5 is not used, or of the so-called cross renchicur type. In this case, the screen is tilted forward about 7.degree., and the two sheets are connected by constructional binding means, or with screws, at 10 to 30 places within the screen. However, the screws are seen within the screen, thus deteriorating the external appearance and the picture quality.
Therefore, the transmission-type screen using two or more sheets including at least the Fresnel sheet 5 and the front sheet 4 has been desired to be able to be tilted forward without screws and to sufficiently withstand the change of the environmental temperature and moisture.